1. Field of the Invention
The present invention relates to implantable medical devices which deliver energy to cardiac tissue for the purpose of maintaining or producing a regular heart rate. Such devices are commonly referred to as cardiac pacing devices and defibrillators.
2. Description of the Related Art
A remedy for people with slowed or disrupted natural heart beating is to implant a cardiac pacing device. A cardiac pacing device is a small electronic apparatus that stimulates the heart to beat at regular rates. It includes a pulse generator, implanted in the patient's chest, which produces electrical pulses to stimulate heart contractions. Electrical leads extend from the pulse generator to electrodes placed adjacent to specific muscles of the heart, which when electrically stimulated produce contraction of the adjacent heart chambers.
Modern cardiac pacing devices adapt their pulse rate to adjust the heartbeats to the patient's level of activity, thereby mimicking the heart's natural beating. The pulse generator modifies that rate by tracking the activity at the sinus node of the heart or by responding to other sensors that monitor body motion and rate of breathing.
Different pacing needs are met by adjusting the programming of the pulse generator and by the location of the electrodes. It is quite common that the leads extend through blood vessels which enter the heart so that the electrodes can be placed in the muscle of the heart chamber requiring stimulation. This requires that the leads extend for some distance through the blood vessels and may necessitate that the leads pass through one or two heart valves. In other patients, patch electrodes are placed on the exterior heart surface with wires extending through tissue to the pacing device. With either type of lead placement, it is important that the electrodes be attached to the proper positions on the heart to stimulate the muscles and produce contractions. Thus it is desirable to properly locate the electrodes for maximum heart stimulation with minimal adverse impact to other physiological functions, such as blood circulation.
Other patients have hearts that occasionally go into fibrillation where the heart has very rapid shallow contractions and, in the case of ventricular fibrillation, may not pump a sufficient amount of blood to sustain life. Administration of a controlled electrical shock to the heart often is required to restore a normal rhythm. A defibrillator often is implanted in the chest cavity of a person who is susceptible to recurring episodes of ventricular fibrillation. Similar to a pacing device, the implanted defibrillator senses the rapid heart rate during fibrillation and applies a relatively high energy electrical pulse through wires connected to electrodes attached to the exterior wall of the heart or to leads in the heart chamber. The defibrillator generates a much more intense electrical pulse than is used by pacing devices which merely stimulate contractions of the heart.
A common heart condition is atrial fibrillation in which the upper chambers, the atria, of the heart quiver instead of beating effectively. Rapid atrial beating produces a corresponding rapid beating of the ventricles. Electrical cardioversion and drugs have been used to restore the heart's normal rhythm. Chronic atrial fibrillation, in which a normal rhythm could not be restored, is commonly treated with medication, such as beta blockers, to slow the rapid heart rate.
Scientific research on dogs discovered that transvenous parasympathic, or vagal nerve stimulation can be employed to slow the rapid ventricular rate induced by atrial fibrillation. In this treatment, an electrode at the tip of a catheter is fed through the blood vessels to a parasympathic nerve stimulation site in the inferior vena cava of the heart. During atrial fibrillation, electrical pulses were applied from an external source through a conductor in the catheter to the electrode, thereby stimulating the site in the inferior vena cava. Specific patterns of stimulation pulses slowed the ventricular rate.